Mass Spectrometry
• Mass spectrometry (MS) is not true “spectroscopy”
because it does not involve the absorption of
electromagnetic radiation to form an excited state.
• MS is very useful for
• Determining a compound’s molecular weight
• Detecting the presence of Br, Cl, and N atoms in a molecule
• Structure determination
• Two things happen in a mass spectrometer
1. A compound is vaporized in a vacuum and then ionized.
2. The masses of the ions are detected and graphed.
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Mass Spectrometry
• The most common method of ionizing molecules is by
electron impact (EI). The sample is bombarded with a
beam of high energy electrons (1600 kcal or 70 eV).
• EI causes an electron to be ejected from the molecule.
• A radical cation is the result.
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Mass Spectrometry
• The initially formed radical cation is known as the
molecular ion (M+•).
• The mass of the M+• is the same as the mass of the
original molecule.
• The M+• is generally very unstable and usually
undergoes a variety of fragmentation reactions.
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Mass Spectrometry
• The resulting fragments may undergo even further
fragmentation often to form radicals and cations.
• The cations are accelerated toward an analyzer, which
separates them based on the mass to charge ratio, m/z.
– Separation Methods include using a magnetic field, time-offlight, ion trapping, and quadrapole.
• Neutral fragments are not detected.
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Mass Spectrometry
• Here is the MS of methane (MW =
16)
• The base peak is the tallest peak in
the spectrum.
• For methane, the base peak is M+•.
• For some molecules, the M+• peak
is not observed in the spectrum.
Why?
• What is the small peak at m/z =
17?
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Mass Spectrometry
• Peaks with a mass of less than M+• represent fragments:
• Subsequent H radicals can be fragmented to give the
ions with a mass/charge = 12, 13 and 14.
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Mass Spectrometry
• MS is a very sensitive analytical method.
• Many organic compounds can be identified:
–
–
–
–
–
–
–
–
Pharmaceutical: drug discovery and drug metabolism,
Organic Synthesis: reaction monitoring, product characterization
Biotech: amino acid sequencing, analysis of macromolecules
Clinical: neonatal screening, hemoglobin analysis
Environmental: water quality, food contamination testing
Geological: evaluating oil composition
Forensic: explosives, illegal drugs
Many More
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15.9 Analyzing the M+• Peak
• In the mass spectrum for benzene, the M+• peak is the base
peak.
• The M+• peak does not easily fragment.
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15.9 Analyzing the M+• Peak
• Like most compounds, the M+• peak for pentane (MW =
72) is NOT the base peak. This is because the molecular
ion fragments easily.
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15.9 Analyzing the M+• Peak
• The first step in analyzing a mass spec is to identify the
M+• peak:
– Tells you the MW of the compound.
– The Nitrogen Rule
• If m/z for the M+• peak is odd, this usually means that there is a
nitrogen atom in the molecule. (Or an odd # of Ns)
• If m/z for the M+• peak is even, then there are no nitrogens. (Or an
even # of Ns)
Chemical Formula: C5H11NO
Molecular Weight: 101
CH3NH2
Chemical Formula: CH5N
Molecular Weight: 31
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Chemical Formula: C3H9N
Molecular Weight: 59
NH2
O
Chemical Formula: C4H4N2
Molecular Weight: 80
N
N
N
H
Chemical Formula: C5H11N
Molecular Weight: 85
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N
N
Chemical Formula: C5H5N
Molecular Weight: 79
Klein, Organic Chemistry 1e
15.10 Analyzing the (M+1)+• Peak
• Recall that the (M+1)+• peak in
methane was about 1% as
abundant as the M+• peak.
• The (M+1)+• peak results from
the presence of 13C in the
sample. The natural abundance
of 13C is 1.1%. Thus approx 1%
of the molecules will have a
MW of M+1.
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15.10 Analyzing the (M+1)+• Peak
• For every 100 molecules of decane, how
many of them will contain one C-13 atom.
• Comparing the heights of the (M+1)+• peak
and the M+• peak can allow you to
estimate how many carbons are in the
molecule.
• The natural abundance of deuterium is
0.015%. Will that affect the mass spectrum
analysis?
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15.11 Analyzing the (M+2)+• Peak
• Chlorine has two abundant isotopes:
–
35Cl=76%
and 37Cl=24%
• Molecules with one Cl have strong (M+2)+• peaks.
• Below is the spectrum of chlorobenzene, C6H5Cl (MW =
112.56)
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15.11 Analyzing the (M+2)+• Peak
•
79Br=51%
and 81Br=49%, so molecules that contain a
bromine atom show equally strong (M)+• and (M+2)+•
peaks. See spectrum of C6H5Br below (MW = 157.0)
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15.12 Analyzing the Fragments
• Analysis of the fragment peaks can often yield structural
information.
• Consider pentane.
– Remember, MS only detects charged fragments.
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15.12 Analyzing the Fragments
• What type of fragmenting is responsible for the
“groupings” of peaks observed?
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15.12 Analyzing the Fragments
• In general, fragmentation will be more prevalent when
more stable fragments are produced.
• Correlate the relative stability of the fragments here
with their abundances on the previous slide.
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15.12 Analyzing the Fragments
• Consider the fragmentation below.
• All possible fragmentations are generally observed
under the high energy conditions employed in EI-MS.
• The most abundant fragments can often be predicted.
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15.12 Analyzing the Fragments
• Alcohols generally undergo two main types of
fragmentation: alpha cleavage and dehydration.
• They often do not display an M+ peak. Instead the
highest m/z is at M – 18.
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15.12 Analyzing the Fragments
• Amines generally undergo alpha cleavage:
• Carbonyls generally undergo McLafferty rearrangement:
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15.13 High Res MS
• High resolution (high-res) MS allows m/z values to be
measured to 4 decimal places. “Exact Mass”
• 12C weights exactly 12.0000 amu. Why?
• All other atoms have known exact masses.
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15.13 High Resolution Mass
Spectrometry
• Why are the values in the table different from those on
the periodic table?
• Imagine you want to use
MS to distinguish between
the molecules below.
• Why can’t you use low
resolution (low-res) MS?
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15.13 High Resolution Mass
Spectrometry
• Using the exact masses and natural abundances for
each element, we can see the difference high-res
makes.
• The molecular ion results from the molecule with the
highest natural abundance.
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15.14 GC/MS
• MS is suited for the identification of pure substances.
• However, MS instruments are often connected to a gas
chromatograph (GC) so mixtures can be analyzed.
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15.14 GC/MS
• GC-MS gives two main
forms of information:
1. The chromatogram gives
the retention time.
2. The Mass Spectrum
3. GC-MS is a great
technique for detecting
compounds such as drugs
in solutions such as blood
or urine and for analyzing
reaction products.
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